Dose-response relationships were examined for the production of hepatic fibrosis and cirrhosis by combined exposure of male Porton rats to alcohol and carbon tetrachloride. Alcohol was administered orally in Lieber-DeCarli liquid diet at levels of 75, 150, or 300 kcal/liter, giving mean daily intakes of 2.29, 4.61, and 8.16 g/kg/day, respectively. Carbon tetrachloride was administered by inhalation at concentrations of 10, 20, or 40 ppm, 6 hr/night, 5 nights/week. Liver biopsies were taken at intervals up to a maximum treatment period of 20 weeks. All four rats that received the high dose of both agents, and 1 of 4 that received the medium alcohol and high carbon tetrachloride treatments, were cirrhotic by 10 weeks. Two of the 4 rats that received the low alcohol and high carbon tetrachloride dose were cirrhotic at 20 weeks. Cirrhosis was not observed in rats that received the low or medium carbon tetrachloride dose, but some degree of hepatic fibrosis was observed in all treatment groups. Severity of fibrosis was significantly associated with both dose of alcohol and dose of carbon tetrachloride received. It is concluded that, in the alcohol-carbon tetrachloride rat model for chronic liver injury, both alcohol and carbon tetrachloride contribute to the response in a dose-related manner. Hepatic injury was observed even when relatively low doses of these agents are administered together.